In geophysical prospecting, seismic operations are frequently used to generate, collect, and analyze Q information about subsurface formations. These operations are usually performed by initiating seismic waves or acoustic signals that travel downward into the earth until they encounter discontinuities in the earth's structure in the form of varying subsurface strata formations. Such discontinuities reflect at least part of the acoustical signals back toward the earths surface. In oil and gas explanation operations, these reflected acoustical signals are recorded and studied to help locate and analyze various subsurface formation for potential oil and gas production.
In oil and gas operations, seismic energy sources such as dynamite or blasting caps are frequently used to generate acoustic signals. In addition, sources such as vibrators or thumpers are used to generate the acoustic signals.
One variation of the typical seismic exploration method is called Vertical Seismic Profiling ("VSP"). VSP is known to be valuable in structural and stratigraphic interpretation of subsurface formations and geological prospecting for oil and gas. In VSP, a geophone or other type of acoustic detector is lowered into a borehole. Acoustic signals are then generated at various ground surface locations away from the borehole. Recordings are made through the geophones at various levels in the borehole.
In VSP, the acoustical signals travel from the signal source through the near ground surface only once on their way to the geophone in the borehole. This results in less attenuation of high frequency waves than occurs for typical surface seismic operations when signals must travel through the near ground surface twice. These higher frequencies give VSP better resolution than surface seismic methods.
A disadvantage of VSP is that numerous offset energy source locations are required to obtain the amount of seismic information necessary to properly study a given subsurface formation. Placement of these offset energy sources is time consuming and expensive. Often the placement of the seismic energy sources, such as dynamite, blasting caps, or large vibrators or thumpers, at a desired location is difficult. Seismic sources must be kept some distances from buildings, dwellings, roads and other structures that would be affected by blasting or the use of dynamite. Also roads to isolated exploration areas may not allow for transporting large pieces of seismic equipment to required locations.
In order to obtain the benefits of VSP in areas where using a seismic source to create acoustical signals from a surface location may be difficult, a modified VSP method referred to as reversed VSP, is used. In reverse VSP, a seismic source is placed in the borehole and geophones or other types of acoustical detectors are laid out on the surrounding ground surface. The surface receivers can be located in positions that would not permit the use of dynamite that are inaccessible to seismic sources such as large vibrators. In addition to being useful in places conventional VSP cannot be used, reversed VSP is capable of obtaining higher quality data than conventional VSP. In reversed VSP operations, receivers can be buried in complicated arrays which improve the frequency content of the reflected signals and reduce noise in the reflected signals. Accordingly, higher frequency and more consistent data can be recorded with reversed VSP than with conventional VSP. The most significant advantage of reverse VSP is that a single downhole seismic source, if used with a large number of geophones at the ground surface, can generate data equivalent to many standard VSP operations with various offsets.
A seismic operation similar to reverse VSP is cross-hole seismology. In cross-hole seismology, a seismic source is lowered into one borehole and a geophone is lowered into a second borehole. The seismic source creates acoustical signals that travel from the first borehole to the second borehole where the signals are measured and recorded. Cross-hole seismology does not require the laying out of surface geophones as is required in reversed VSP. Because the acoustical signals do not have to travel through the near ground surface, seismic data is produced having high resolution and a high signal-to-noise ratio. Cross-hole seismology is most generally used in producing fields, where existing boreholes may be used to provide additional information about previously discovered reservoirs.
Various downhole energy sources are available for use in reversed VSP and cross-hole seismology. As stated previously some of the methods for generating acoustical signals include the use of explosive blasting caps, sidewall coring guns, and perforating guns. Although these methods could provide an energy source of acceptable intensity for generating acoustic signals, a blasting cap allows only a single explosion for each downhole trip and sidewall coring and perforating guns may damage the casing of the borehole. Currently, individual explosive charges or series of explosive charges without the damaging effects of the above mentioned guns are frequently used. These charges are electrically detonated from the ground surface by a seismic crew through a standard seven conductor wireline cable. However, the use of a standard wireline cable, limits the number of individual explosions available to be fired on a single downhole trip with a downhole source.
Another downhole seismic source currently used is an air gun. An advantage of an air gun is that it may be moved up and down the borehole and repeatedly fired at various positions on a single downhole trip. However, an air gun has mechanical limitations and use restrictions that can make its operation and handling difficult. The firing control line and high pressure air hose are very bulky and can be difficult to operate in a deep borehole. In addition, a downhole air gun usually produces less acoustical energy than a 10 gram explosive charge of a standard pentaerythritol tetranitrate (PETN) explosive. Because of this relatively weak energy source level, air guns are usually used only for cross-hole seismology but not for reversed VSP.
Another disadvantage of using an air gun is that air guns produce more tube-wave energy in the borehole than do other explosives. Existence of such tube waves complicates data processing and interpretation of the recorded data. Additionally, air bubbles are produced during operation of an air gun which change the acoustical properties of the mud column, which complicates signal processing.
Finally, an air gun's performance may be adversely affected by large hydrostatic pressures such as when the gun is operated at significant depths.
The downhole energy source that generates the most desirable acoustical signals is an explosive charge, such as is obtained in firing a sidewall coring gun or perforating gun. Explosive shot arrangements without the damaging effects of coring guns or perforating guns are commercially available. The firing of these explosive shot arrangements can be controlled at the ground surface through standard seven conductor wireline cable. A limitation in using standard surface firing control equipment with a standard seven conductor cable is that a maximum of 14 individual or group shots can be fired before a downhole firing arrangement must be removed from the borehole and reloaded. The reason for this limitation is that present methods use the seven lines to carry current to the explosive and uses only the armor line as a return to complete the circuit that allows current to flow. This method is limited because each line can have only one positive or negative signal. With seven lines, there are possibilities for current flow to generate only 14 signals. Therefore, using only the armor line as stated above only 14 explosives can be fired. Since in typical reversed VSP and cross-hole seismology operations, the firing of hundreds or thousands of shots might be required to generate the necessary amount of seismic information, a downhole firing apparatus using a standard surface firing control arrangement would require many downhole trips. Such numerous trips are time consuming, expensive, and prevent quick gathering of large amounts of data.
U.S. Pat. No. 4,895,218 (Chen et al.) addresses many of the above mentioned problems by providing a downhole seismic source capable of selectively firing numerous explosives in a downhole arrangement to produce seismic waves in a subsurface formation. The downhole source is capable of firing numerous prewired explosive charges. The downhole source uses a self-contained firing circuit that receives select signals and fire signals from a seismic crew at the ground surface through a standard seven conductor wireline cable to detonate the explosive charges. The downhole seismic source is capable of generating numerous signals in a given downhole trip through the firing of many individual explosive charges. However, this device requires complex equipment in order to operate. There exist a need for a simple and safe way of individually firing more than 14 small (typically 7 gram) explosives in a borehole via a standard seven conductor wireline. The principal application of these small explosives would be as an effective downhole acoustic source for borehole geophysics.